Recreational and industrial vehicles are prevalent in today's world. Examples include golf carts, forklifts, and airport transport and luggage handling carts. Because electric vehicles create less pollution than internal combustion (i.e., gasoline and diesel powered) vehicles, they are an environmentally friendly, and increasingly acceptable, alternative.
As shown in FIG. 1, electric vehicles are typically powered by a battery pack comprised of a plurality of rechargeable batteries (or “cells”) 100. The battery pack cells 100 are housed in a battery pack case (or “tray”) 102. The cells 100 are usually connected in series by way of electrical connectors 104. The battery pack case 102 is typically semi-permanently mounted on or inside the electric vehicle.
A necessary operational aspect of electric vehicles is the periodic recharging of the battery pack. In some applications the battery pack may be recharged without having to remove the battery pack from the vehicle. However, in other applications the depleted battery pack must be removed and replaced with a fully charged replacement battery pack. In factory operations, for example, the electric vehicles (typically forklifts) are powered by high-capacity batteries. High-capacity batteries have amp-hour ratings of 1000 Amp-hrs or more, and require six to eight hours of charging to restore the battery to full charge. Hence, to avoid rendering the vehicle unavailable for use during the six to eight hours needed to recharge the depleted battery pack, the depleted battery pack is typically lifted out of the vehicle and replaced with a fully charged replacement pack. Because the battery packs can weight up to 4,000 lbs, special hydraulically powered lift machines are used to complete the battery pack swapping operation.
In recent years, engineers have developed what is known as “fast charging” technology. Fast charging reduces the recharge time of a 1000 Amp-hr battery, from the typical six to eight hours required using conventional battery charging techniques, to about an hour. Fast charging thereby allows recharging to be performed, for example, during an operator's lunch break, or during other opportune times when the vehicle may not be needed. For this reason, fast charging technology is sometimes referred to as “opportunity charging”. Fast charging also eliminates the need to repeatedly swap out and replace depleted battery packs with charged battery packs.
While fast charging improves operational efficiencies, its use generates temperatures and thermal gradients in a battery pack, which if not properly controlled contribute to degraded performance and a shortened lifespan of the battery pack. FIG. 2 shows a graph of the effective internal resistance and heat generation observed in a typical thirty-six-volt industrial battery at different inrush currents and states of charge (SOC). The “Fast Charge Zone,” which is defined by the lowest resistance region, is located between about 20 and 70% SOC. The graph shows that in the Fast Charge Zone, fast charging at 600 amps generates up to ten times as much heating as conventional charging at 200 amps. This excessive heating results in significantly higher temperatures in the fast charged battery pack.
Heating of cells of a battery pack, whether attributable to fast charging or heavy-load use, is exacerbated by the fact that the various cells of the battery pack are typically arranged in a grid pattern and housed in a battery pack case, similar to that shown in FIG. 1. This substantially enclosed configuration does not allow for any significant cooling paths, especially for cells disposed in the center of the pack. While the cells closest to the metal case can cool to some extent through the case wall, the center cells have to cool through their neighbor cells or by radiating heat from their top surfaces.
Because the center cells of a battery pack endure higher temperatures than cells forming the periphery of the battery pack, the center cells are plagued with diminished performance and are even prone to fail more often compared to the peripheral cells. FIG. 3A shows cell voltages of a battery pack of eighteen cells (which are arranged as shown in FIG. 3B) during discharge at 20% SOC, before and after equalization (EQ) of a battery pack that has undergone fast charging during a life cycle test. Overall, it is seen that the cell voltages of all cells are higher after EQ compared to before EQ. However, even after EQ the cell voltages of the “middle-cell group,” which comprises cells 8 through 11, tend to remain lower than the cell voltages of peripheral cells 1 through 6 and 13 through 18.
FIG. 3A also shows that temperatures of the cells of the middle-cell group are significantly higher than the temperatures of the peripheral cells after EQ. These temperature differentials for center cells of a typical eighty-volt industrial battery pack during fast charging are illustrated in FIG. 4. As can be seen, there is up to a thirty-degree temperature gradient between cells in the center of the battery pack and cells that form the periphery of the battery pack.
As the foregoing demonstrates, without adequate cooling a battery pack is beset with reduced capacity and run time. Cell-to-cell imbalances can also lead to over-discharging during use and overcharging during recharging, both of which further affect the performance and lifespan of the battery pack. Therefore, there is a need for methods and apparatus for providing adequate cooling to battery packs, particularly, but not limited to, those used in industrial applications.
The operational efficiencies gained by fast charging introduce additional problems beyond that of just thermal and ventilation management. For example, the motor drive systems of most electric vehicles are not designed to withstand the high voltages employed by fast charging techniques. Accordingly, there is also a need for methods and apparatus to prevent these high voltages from being coupled to the motor drive system of an electric vehicle while the battery pack is being fast charged.
There are also safety and damage concerns relating to connecting a charger to the charging connections of a battery pack. Typically, a battery pack is located under a hood of the electric vehicle, and has charging connections that are not easily accessible by an operator. These undesirable characteristics expose the operator to the possibility of coming into contact with battery acid and/or increasing the risk of electrical shock when the operator is connecting charger connectors to the battery pack. Further damage and injury can result when the operator inadvertently fails to disconnect the battery charge connector from the charging connections of the electric vehicle, but then drives the vehicle away from the charger. Accordingly there are also needs for improved access and safety measures for use in charging battery packs of an electric vehicle. These needs would preferably be met by not having to make any modifications to the electric vehicle.
Finally, satisfactory solutions to integrating fast charging technology into electric vehicles are not available in the prior art. Rather, prior art solutions are ad hoc and require that modifications be made to the vehicle. For example, holes must be drilled and tapped to mount fans for cooling and to configure, route and mount fast charge connectors to the vehicle. Holes must also be cut into the battery compartment to allow the fast charging battery cables to pass through to the outside. In addition to the expense and tedium required to make such modifications, modifications themselves are undesirable since they can potentially void the vehicle's warranty and/or UL listing. Modifications also result in a reduction in the resale value of the vehicle, or a possible financial penalty being assessed against the lessee of a leased vehicle. Further, the ad hoc nature of prior art approaches results in a lack of uniformity, failing to provide a unique, integrated solution that can be consistently and successfully performed to accommodate fast charging technology without the need for operator intervention. Accordingly, there is a need for methods and apparatus for accommodating fast charging technology that do not require having to make modifications to the vehicle.